This invention relates to a sequence network or filter used for the protection of an electrical transmission line and generators, and more particularly, to a sequence network having phase shift networks on its input stage which provides compensation for frequency variations of the applied input signal so as to develop a substantially accurate desired sequence component in spite of input frequency variation.
As is known, phase sequence networks are responsive to selected symmetrical components of current in a polyphase alternating current electric power circuit. The theory of symmetrical components is well understood by those skilled in the art of electric power transmission and distribution. In accordance with this theory, the phase currents or voltages in any unbalanced three-phase alternating-current electric circuit can be resolved into three sets of symmetrical, balanced current or voltages vectors known respectively as positive sequence, negative sequence and zero sequence component. The negative and zero sequence components of circuit current and voltage are not present under balanced circuit conditions. A more detailed discussion of the theory of symmetrical components can be found in the textbook, Symmetrical Components, by Wagner and Evans, published by McGraw-Hill in 1933.
It has been known that certain circuits, called sequence networks or filters, can be connected to an electric-power system to provide an output signal that is proportional to the magnitude of the negative sequence components of current. These sequence networks or filters are particularly useful in the protective relaying art as they will sense the presence of negative sequence components of current which flow when the power system becomes unbalanced due to abnormal or fault conditions. It has also been known in the protective relaying art that it is desirable to have a sequence network that senses the positive sequence components of current of the power system. A sequence network commonly utilized in protection of a high voltage transmission line and generators is described in U.S. Pat. No. 4,342,062.
Typically, sequence networks derive their negative or positive sequence components from the voltage or current quantities of a power source having a rated frequency supplying a three phase transmission line. A difficulty with prior sequence networks is that their accuracy may be seriously degraded by changes in the input frequency of the power source. Such frequency changes sometimes occur on electrical power distribution systems where these sequence networks are commonly used. For example, assuming the sequence network desires to sense the negative sequence component related to the three phase transmission line and further assuming only the positive sequence component is applied to the network, for this condition it is desired that no output quantity be developed by the sequence network. However, if the input frequency of the power source changes, an output quantity of the sequence network may occur even though the input signal to the sequence network still contains only a positive sequence component. The output is obviously an error since it indicates a negative sequence component where none exists. This error occurs because the phase and amplitude characteristics of the phase shift networks typically used on one or more phases to obtain phase shifted quantities relative to the other phase or phases vary with deviations from the rated frequency of the power source. Further, this variation may be such that undesired sequence components may no longer be cancelled. It is desired that a sequence network be provided for developing positive or negative components whose performance is substantially constant even though the frequency of the power source supplying the transmission line may be a variable quantity.
One use of a desired sequence network may be in the "art of instrumentation". The term "art of instrumentation" is meant to represent the art of developing in a very accurate manner a function which may be monitored and measured, and which function is an indication of a particular condition of an overall system. For example, in the field of power generation the unbalanced currents, indicative of a power loss quantity, developed within the power generator may be manifested by the presence of negative sequence current. If this negative sequence current is accurately sensed by a sequence network the output of the sequence network may be routed to an external instrumentation circuit, which, in turn may develop an accurate quantity indicative of a primary characteristic, that is the power loss quantity, of the power generator.
Accordingly, one object of the present invention is to provide a sequence network for accurately developing positive or negative sequence components over a limited but adequate range of frequency variations.
It is another object to accurately develop the positive or negative sequence components so that they may be further used for instrumentation purposes.
These and other objects of the present invention will become apparent to those skilled in the art upon consideration of the following description of the invention.